Video signals may include blanking intervals, such as horizontal blanking intervals and vertical blanking intervals. Blanking intervals are located between active video segments, of the video signal, which contain video content for display. But the blanking intervals themselves do not include video content for display.
In digital video transmission systems, image, audio and other data are formatted according to well established standards. However, some of these standards make only very limited provisions for the detection and correction of transmission errors within the system. Therefore, these systems are vulnerable to impairments caused by increasing data rates and/or increased transmission distances. Errors in transmission ultimately degrade the image quality at the receiver, corrupt ancillary data and can cause the receiver to lose video timing synchronization.
Certain characteristics of a video system, such as transmission distance, quality of the transmission channel, operational environments or data rates directly affect the probability and rate of errors at the receiver. In turn, the overall error rate directly affects the image quality, the data integrity of the ancillary data and the video timing synchronization of the transmitted video signal. Thus it becomes increasingly difficult to maintain constant video quality over increasing transmission distances and at the higher data rates demanded by newer video standards.
FIG. 1 is a schematic diagram of a prior art video communication system 10. The system 10 includes a video transmitter 11 that transmits a video signal 12 to a video receiver 13. The receiver 13 generates, from the transmitted video signal 12, a raster signal 14, which can be analog, that is channeled to a display screen 15, such as a cathode ray tube of an analog television.
For historic reasons, modern digital video systems have frame structures that retain properties of the analog video systems that they replaced. A video signal is broken up into several different components for transmission. Each transmitted image frame is first broken into horizontal lines (scan lines), transmitted from left to right, top to bottom. In older cathode ray tube technologies, it was necessary to disable the electron beam (called blanking) after drawing a scan line in order to return the beam to the left side of the display.
FIG. 2 is a schematic diagram of a video frame 21 (image) that is displayed by the display screen 15. The frame 21 comprises a vertically-extending series of horizontal lines 22 (scan lines). To generate the frame 21, a beam of the cathode ray tube starts by impinging a screen phosphor at the top left corner 23 of the frame 21 (and of the display 15) and generates the successive horizontal lines 22 in a zigzag fashion. Each scan line 22 is generated from the left end 24 of the scan line 22 to the right end 25 of the scan line 22, and is followed by a horizontal blanking interval 26 of time during which the beam is turned off (disabled, blanked) while it returns to the left end 24 of the frame 21 to start the next scan line 22. After the beam reaches the bottom right corner 27 (rightmost end of the last horizontal line) of the frame 21, there is a vertical blanking interval 28 of time during which the beam returns to the top left corner 23 of the frame 21 (screen 15) to start producing the next frame in the video signal 12.
FIG. 3 is a schematic diagram of the transmitted signal 12. In this diagram, time increases from left to right, as indicated by an “time” arrow above the diagram pointing to the right. The transmitted signal 12 includes a series (succession) of video lines 30, one for each scan line 22 of video. Each video line 30 includes an active video segment 31 (video content segment) that contains data defining how the respective scan line 22 is to be rendered on the screen 15, and a horizontal blanking segment 32. Accordingly, neighboring active video segments 31 are separated by one of the blanking segments 32, and the blanking segments 32 are interspersed among the active segments 31. The transmitted signal 12 includes a vertical blanking segment 33 temporally located between one frame's data and the next frame's data. Accordingly, data for neighboring frames are separated by a vertical blanking segment 33. The active video segments 31, horizontal blanking segments 32 and vertical blanking segments 33 shown in FIG. 3 are temporally aligned respectively with the frame's scan line 22, horizontal blanking intervals 26 and vertical blanking interval 28 shown in FIG. 2.
In the transmitted signal 12 in FIG. 3, each active video segment 31 is immediately preceded by a start of active video (SAV) segment 34 and immediately followed by an end of active video (EAV) segment 35. Alternatively stated, each horizontal blanking segment 32 is immediately preceded by an EAV segment 35 and immediately followed by a SAV segment 34. The SAV and the EAV together comprise a timing reference signal (TRS).
One example of the receiver 13 is an analog television that receives the raster-based video signal 12 illustrated in FIG. 3 and renders images (frames) on a cathode ray tube using the raster-based procedure illustrated in FIG. 2. Another example of the receiver 13 is a digital video receiver, such television with a flat screen LED-based or LCD-based display, that receives the raster-based signal 12 described above, and renders the images (frames) on a display using a procedure other than the raster scan procedure described above, and therefore does not require a horizontal or vertical blanking interval for enabling a beam to return to a prior location, but requires the blanking intervals instead for proper signal formatting. With either type of receiver (i.e., with cathode ray tube and without), the rate of errors in the signal arriving at the receiver increases as data rate and transmission distance increase.